1. Technical Field
The present invention relates to electrosurgical generators and, more particularly, to electrosurgical generators having specific utility in arthroscopic surgical procedures.
2. The Prior Art
The prior art contains a variety of electrosurgical generators which utilize high frequency electrical signals to effect surgical cutting and/or coagulation. These signals are generally referred to as cutting signals, coagulation signals, or blend signals, the latter being formed by combining both the cutting and coagulation signals. The coagulation signals, in turn, may be subdivided into fulgurate and desiccate signals, depending upon the intended operating mode of the machine. Such signals are applied to a patient, conducted through the patient's body, and returned to the generator via a ground path.
The cutting signal is a high frequency signal which serves to cut through tissue when applied to the patient. An electrosurgical electrode is used to apply the electrical energy to defined and concentrated points of a patient's body. Cutting is accomplished by the concentrated application of high frequency electrical energy which effectively destroys the body cells directly beneath the electrosurgical electrodes. Coagulation signals are intended to produce coagulation by shrinking vessel walls. Typically, such coagulation signals are pulses of energy having a damped sinusoidal wave form. Coagulation signals may be viewed as causing cell dehydration to produce coagulation rather than destroying cells in the fashion of cutting signals. The blended signals are formed by combining the cutting and coagulation signals and are useful for accomplishing cutting and coagulation simultaneously. Alternating periods of each signal may be employed to form the blended signal.
There are many types of electrosurgical generators available in the prior art which are intended to function for general purpose surgery. Such generators are configured to efficiently cut into tissue impedances which range between three hundred to five hundred Ohms, and have output power levels which are typically in excess of three hundred watts. Such generators, however, are not efficient for performing surgery on the joints of the human body. One reason for this is the impedance presented by bony substances, cartilage, meniscus, etc., which impedance is considerably higher than that for which most general purpose electrosurgical generators are designed. When the impedance of the generator is not properly matched to the impedance of the human joint at the surgical site, the generator does not efficiently transfer energy to the surgical site and, therefore, relatively high power settings must be achieved to accomplish the desired surgical effect. These high energy settings can result in undesirable tissue damage, such as necrosis, in the vicinity of the operating area.
Another consideration affecting the inapplicability of most prior art electrosurgical generators for arthroscopic surgery relates to the slow start cutting characteristics of such generators. Specifically, arthroscopic knee surgery is performed in an irrigating media of water and requires that the electrosurgical generator be capable of cutting and coagulating under these conditions. When prior art electrosurgical generators are employed in wet fields or under water, they exhibit slow start cutting characteristics which surgeons generally refer to as a "delay" or "drag in cut". This effect can be overcome by the use of high power settings; however, after the cut is initiated, the energy delivered to the surgical site is much higher than required to sustain the desired cutting effect. One electrosurgical generator on the market provides a rapid start option in order to eliminate this cutting delay. However, the rapid start feature provided in such generator degrades the normal operating capabilities of the system.